Joining Castro at the campaign kickoff was his twin brother, Democratic Rep. Joaquin Castro, chairman of the Hispanic congressional caucus and a frequent Trump critic.
Acknowledging that his run for the White House was a long shot, as a growing number of high-profile Democrats are expected to enter the race, Castro referred to his hometown of San Antonio.
One of Castro's major problems is that he really doesn't have much of a resume compared to some of the heavier hitters we're expecting to get into the race over the next few months.
"We're going to make sure that the promise of America is available to everyone", Castro said before a cheering crowd on Saturday in his home town of San Antonio, Texas.
Castro, the former mayor of San Antonio, had been considering a bid for almost two years and announced a presidential exploratory committee in December.
Castro was reflective days before his announcement, acknowledging how his wife's support and mother's political work helped him get to this point. Former Rep. John Delaney (D-Md.) and Rep. Tulsi Gabbard (D-Hawaii) have already announced definitive intentions to run for president in 2020. Sen.
Castro is the first Mexican-American and first San Antonio native to seek the White House.
The impasse over paying for a border wall that Trump made a central part of his 2016 campaign has led to the partial federal closure. There are about 700,000 young "Dreamers" who were brought into the USA illegally as children; advocates want to provide them with protection deportation and a chance to apply for citizenship.
Mr Castro would be the first identical twin elected president.
Castro, whose grandmother came to the USA from Mexico in the 1920s, would be vying to become the party's first Hispanic-American nominee amid a field of that could swell to more than 20 candidates.
That group of hopefuls is starting to take shape even though the first primary elections are more than a year away.
So, too, is former Rep. Beto O'Rourke, D-Texas., who could be more of a challenge. Elizabeth Warren of Massachusetts, Kamala Harris of California and Cory Booker of New Jersey have moved ahead of Castro in name recognition. Ted Cruz turned O'Rourke into a national Democratic celebrity. He was mayor of San Antonio from 2009 to 2014 - the youngest mayor of a top-50 American city.
Castro was raised by a local Latina activist.
Castro may not be the only Texan in the race, however. It wasn't long before Democrats nationally embraced him as a star in the making, particularly one from Texas, where a booming Hispanic population is rapidly changing the state's demographics and improving the party's fortunes. He was also famously considered a frontrunner to be former Secretary of State Hillary Clinton's running mate in 2016, but she ultimately went with Senator Tim Kaine.
Urging the enthusiastic crowd to look around the blue-collar neighborhood where he grew up, Castro said, "There are no front-runners that are born here, but I've always believed that with big dreams and hard work, anything is possible in this country".
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